Add Copilot SDK Java instructions file

Add instructions/copilot-sdk-java.instructions.md modeled after the
C# equivalent in github/awesome-copilot. Covers client initialization,
session management, event handling, streaming, custom tools, BYOK,
permissions, and common usage patterns for Java developers.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: brunoborges